Fluid sensor cap assembly

ABSTRACT

Membrane type polarographic oxygen sensors are constructed to be uniformly rechargeable with electrolyte by introducing a selected quantity of electrolyte into a tubular cap for reception at the electrode side of a membrane in the cap; the cap is then assembled into the sensor body in such manner as to bring the electrolyte into contact with the cathode tip, and into a predetermined volume space adjacent the tip for current carrying purposes.

United States Patent Doniguian 51 3,666,650 51 May 30,1972

[54] FLUID SENSOR CAP ASSEMBLY [72] Inventor: Thaddeus M. Doniguian,Laguna Beach,

Calif.

[73] Assignee: I.M.I., Division of Becton, Dickinson and Company,Newport Beach, Calif.

[22] Filed: May 6, 1970 21 Appl. No.: 35,187

52 us. Cl ..204/19s P, 220/39 51 Int. Cl. ..G0ln 27/30 [58} Field ofSearch ..220/3 9; 204 195 P, 1 T

[56] References Cited UNITED STATES PATENTS 3,21 1,638 10/1965 Halvorsen..204/195 3,445,369 5/1969 Porteret al ..204/l95 3,518,179 6/1970Bleaketal ..204/l95 Primary Examiner-G. L. Kaplan Attorney-White,Haefliger and Bachand s7 ABSTRACT 3 Claims, 5 Drawing Figures FLUIDSENSOR CAP ASSEMBLY BACKGROUND OF THE INVENTION This invention relatesgenerally to devices for sensing the quantities of substances, such asgas molecules, present in a composition. More specifically it concernsimprovements in the recharging of membrane type polarographic oxygensensors.

Present sensors of the above type must frequently be recharged withelectrolyte, a process which necessitates removal and cleaning ofmultiple components followed by securing of an oxygen passing membraneabout a cathode on which fresh electrolyte has been placed. Thisprocedure frequently results in undesired loss of electrolyte from thecathode zone, which reduces the life of the recharged sensor; also itcan lead to unwanted error in oxygen concentration readings, and it istime consuming and tedious and requires considerable manual agility.

SUMMARY OF THE INVENTION It is a major object of the invention toprovide apparatus and method characterized as overcoming the aboveproblem stemming with known recharging techniques. Basically, the methodof the invention contemplates securing the membrane to a tubular capprior to-its connection onto the sensor body; introduction of a selectedquantity of electrolyte into the cap for reception at the electrode sideof the membrane; and assembly of the cap onto the sensor body in suchmanner as to bring the electrolyte into contact with the cathode tip,the assembly also causing introduction of the electrolyte into a spaceof predetermined volume adjacent the tip for current carrying purposes,unwanted loss of electrolyte from that space being prevented. As aresult, uniformity of electrolyte recharging is achieved, and in anexpeditious and simple manner. I

In its apparatus aspects, the invention contemplates the provision of acap sized to be assembled over the electrode and onto the sensor body,the cap having a tubular portion to extend about the electrode in spacedrelation to the electrode, a membrane attached to the cap to extendacross the interior of the tubular portion in such position as toreceive displaceable electrolyte to be brought into contact with theelectrode tip and into the space between the electrode and cap tubularextent in response to cap assembly onto the sensor body, the membranecharacterized as capable of passing a selected gas molecule such asoxygen.

As will appear, the membrane may be secured to the cap bore as by meansof a ring providing a stop presented to engage the sensor structure andlimit assembly of the cap onto the body when the membrane has desiredproximity to the cathode tip, a thin film of electrolyte remainingbetween that tip and the membrane. Further, the membrane typicallyprojects generally conically free of a shoulder extending about theelectrode such as a cathode, thereby to form with the cathode andtubular cap an annular space of predetermined volume into whichelectrolyte is received in response to the assembly of the cap onto thebody structure. That shoulder may with advantage be defined by aninsulator surrounding the cathode.

These and other objects and advantages of the invention, as well as thedetails of an illustrative embodiment, will be more fully understoodfrom the following description and drawings, in which:

DRAWING DESCRIPTION FIG. 1 is an exploded view of a conventional sensorshowing the components involved in recharging;

FIG. 2 is an exploded view of a sensor incorporating the invention, andshowing components involved in recharging;

FIG. 3 is an enlarged vertical elevation taken in section to show theFIG. 2 components assembled on the sensor body;

FIG. 4 is a section taken on line 44 of FIG. 3; and

FIG. 5 is an enlarged fragmentary elevation showing the FIG. 3 cathodetip and membrane in assembled relation.

DETAILED DESCRIPTION Referring first to FIG. 1, a known membrane typepolarographic oxygen sensor is shown in disassembled state to illustratecertain problems that arise upon recharging during usage. That. sensormay be as described in US. Pat. No. 2,913,386 to Clark, and include abody 10, an anode 11 projecting at the right end of the body, andcathode 12'projecting from the anode. The anode may for example besilver surfaced, and the cathode may consist of a gold rod embedded in atube 112 of insulative material.

At the right of the cathode in FIG. 1 there are illustrated an oxygenpassing membrane 13, which may for example consist of a disc ofpolyethylene, an O-ring seal 14, a spacer ring 15, a second O-ring 16and a cap 17. These elements are removed from the body 10 when the cap17 is unscrewed from the body threads 18, for recharging purposes. Thelatter process consists of removing and cleaning the cap 17 and elements14-l6. The membrane 13 is discarded and the cathode 12 and anode 1 1along with threads 18 are cleaned. Fresh electrolyte in the form of agel (as for example aqueous potassium chloride solution) is then appliedto the cathode and anode, after which a fresh membrane 13 is placedacross the cathode, folded into conical shape over the anode shank andsecured into the groove 19 on the anode as by means of O-ring 14. Thespacer and O-ring 16 are then fitted over the membrane and the cap 17screwed in place on thread 18. 7

Even under the most ideal conditions, it is difficult to achieve a longlasting charge, since in the process of securing the O-ring 14 onto themembrane much of the electrolyte is forced out from beneath themembrane. In this regard, insufficient electrolyte corresponds toundesirably reduced life of the charge, life referring to the active usetime prior to development of electrolyte depolarization producinginaccurate oxygen readings. Also, the disassembly, cleaning andreassembly of the many parts is at best tedious, time consuming, andreassembly does not always result in a uniform or proper recharge, sothat the useful life of a given charge will vary considerably, andunpredictably, which can be dangerous during surgery where accurateoxygen monitoring is critical.

Referring now to FIGS. 2-5, the pre-assembled membrane and cap indicatedat 30 overcome the above problems. The tubular cap 31 is sized to beassembled over the electrodes (cathode and anode 12 and 11), with skirt32 having interior threading 33 screwed onto the body thread 18. Anoxygen molecule passing membrane 34 has an outer annular portion 34asecured to the cap, as for example by a ring 35. If desired, themembrane portion 34a may be preliminarily attached to the ring to form asubassembly which is then fitted with some interference into the capbore 36 and to endwise engage the cap step shoulder 37. When the cap isassembled onto the thread, the opposite stop end of the ring 35 mayengage the body shoulder 10a to limit the screw-on assemblytopredetermine the degree of forcible contact of the tip of electrode(cathode) 12 with the membrane, as seen in FIG. 3.

Prior to such assembly, the anode and cathode are cleaned, and ameasured amount of electrolyte is inserted into the new cap assembly asindicated by the arrow 40 in FIG. 2 extending from the electrolytedispenser 41. Such electrolyte is received through the cap open end intothe conical zone 42 formed by the membrane, i.e. for most advantageouscontact with the electrodes upon assembly of the cap onto the body, andfurthermore for controlled introduction of a predetermined amount ofelectrolyte into the space 43 between the electrodes to further theobjective of charge life predictability.

In this regard, an annular conical shoulder 44 is presented, as forexample by insulative material 112, to surround the cathode 12 in spacedrelation to the cathode tip 12. During assembly, the membrane 34 urgesthe electrolyte into contact with the electrode tip and into the spaces43 as described and space 47 between ring 35 and anode 1 l, theelectrolyte being indicated at 46 in FIG. 5. Spaces 43 and 47 becomingfilled with electrolyte upon assembly, any excess is then squeezed pastbody shoulder 10a and into the outer space 10b about the body part 113.Accordingly, since the electrical currentpassing extent of theelectrolyte is in spaces 43 and 47, the filled volume of which iscontrolled by the assembly of the cap to the body, the useful life ofthe electrolyte is automatically determined with optimum and repeatableuniformity. Also, this larger volume provides adequate anode exposureand quantity of electrode for extended life.

Note that the membrane projects, in FIG. 3, generally conically at 34!)free of the tube 1 12 from an annular locus of engagement ornear-engagement with shoulder 44. FIG. indicates that a very thin film46a of electrolyte remains between the membrane nose and the electrodetip 12 and also between the membrane and the tip of tube 112, despitepredetermined forcible stretching of the membrane. Assuming oxygendetermination, the elecn'olyte provides for the flow of ions inelectrareduction and electro-oxidation processes in accordance with theoverall equation:

where H represents a hydrogen ion, 0 represents an oxygen atom, and erepresents an electron. Suitable electrolytes are known in the art. Theelectrical current carrying capacity of the cell will vary in directproportion to the quantity of oxygen passing into the electrolyte. FIG.2 illustrates the use of circuitry 60 operable to apply a given voltagebetween the anode and cathode leads 61 and 62, and to read at 63 thecurrent flow which varies with the quantity of oxygen passing throughthe membrane from the side 64 thereof and into the electrolyte film 46a.in this regard, it is contemplated that the invention is usable in otherapplications to sense other gas molecules.

Additional advantages include the ease of recharging without need formanual agility, the disposability of the caps, the ease of preassemblyof the membrane to the cap, and the simplicity of the apparatus andmethod.

Element 70 in FIGS. 1 and 2 is a rear cap attached to body 10, and fromwhich a cable 71 extends, the cable containing leads 6] and 62.

Circuitry 60 may incorporate suitable temperature compensation, such asa temperature sensitive resistance (Le.

thermistor) used with the cathode 12 so that the temperature of thatresistance is the same as that of the fluid being tested at the side 64of the membrane. Such compensation eliminates the effect of temperaturechange upon the current flow read at 63.

' lclaim:

1. For use with sensor structure comprising a body, and electrode meansincluding a projecting electrode carried by the body, the electrodehaving an exposed blunt tip, the combination comprising i a. a cap sizedto be assembled over the electrode and onto said body, the cap having atubular portion to extend about the electrode in spaced relation theretowhen said electrode is inserted forwardly through one open end of thecap, and a membrane extending across the interior of said tubularportion in such position as to receive displaceable electrolyte at oneside of the membrane to be brought into contact with said electrode tipand into the space between said electrode and cap tubular portion inresponse to said assembly of the cap, the membrane characterized aspassing certain gas molecules, there being a ring securing the membraneto the cap bore, a first annular portion of the membrane retainedbetween the ring outer surface and the cap bore, and a second annularportion of the membrane retained between the ring forward terminal and arearward facing annular step shoulder of the cap, that portion of themembrane extending inwardly of said second portion projecting forwardlyof the ring and being openly exposed rearwardly to said one open end ofthe cap to receive said electrolyte, and to centrally receiveapplication of force transmitted by the electrode tip for predeterminedtensioning of the membrane.

2. The combination of claim 1 wherein said membrane is an oxygen passinmembrane.

. The com mation of claim 2 mcludm a sto on the rm g P g

2. The combination of claim 1 wherein said membrane is an oxygen passingmembrane.
 3. The combination of claim 2 including a stop on the ringwithin the cap presented to engage said sensor structure and limit saidassembly of the cap thereto when the membrane is brought into forcetransmitting relation with the shoulder.